Effects of quenching-tempering heat treatment on microstructural evolution and fracture behavior of microalloyed high strength suspension spring 55SiCrVNb were investigated. The results showed that an optimal combination of mechanical properties was obtained at the heat-treatment of oil quenching from 900 °C and tempering at 400 °C. Specifically, the ultimate tensile strength, yield strength, elongation and area reduction reached 2021 MPa, 1826 MPa, 10.3% and 42.7%, respectively. With the increasing austenitizing temperature, the grain size increased monotonically with more carbides dissolved in the matrix, and the strength first increased significantly and then decreased slowly. Furthermore, the fracture behavior transformed from ductile fracture to brittle fracture. In addition, the strengths were weakened dramatically as the tempering temperature increased but the elongation and area reduction were enhanced. Three kind of carbides are identified at different tempering temperatures, namely the coherent M2.5C and MC carbides gradually mature into large-sized non-coherent M3C and M7C3 carbides. Moreover, the dislocation reduction, lath boundary and twin martensite decomposition and carbide coarsening were exacerbated at higher tempering temperature while the fracture behavior changed from brittle fracture to ductile fracture.

Effect of quenching and tempering temperature on microstructure and tensile properties of microalloyed ultra-high strength suspension spring steel

In this study, the micro-alloying of rare-earth yttrium (Y) was employed to improve the fracture behavior and work-hardening ability of H13 die steel. The improved work-hardening ability was attributed to modification of Cr23C6 and VC carbides by the addition of Y. The strip-typed Cr23C6 carbides originally distributed along the grain boundary were interrupted and tended to spheroidize with increasing Y content. Compared with 0Y–H13 steel, the quantity of spherical VC carbides significantly increased in 0.013Y–H13 steel. A higher-density dislocations and intensive interaction between precipitates and dislocation led to a higher work-hardening exponent. Additionally, fractographic studies revealed that the fracture mode transition from cleavage failure to ductile rupture with increasing Y content. Moreover, excess Y content (0.044%) resulted in the appearance of inclusion clusters in dimples, which were detrimental to fracture and work-hardening behavior.

The carbides, tensile properties, and work-hardening behavior of annealed H13 die steels with varied yttrium contents

The boriding of 4Cr5MoSiV1 steel was performed in the temperature range of 860°C–980 °C to examine the influence of boriding conditions on the boride layers The experimental results show the formation of FeB and Fe2B layers with the predominant saw-tooth morphology The boride layer depth increases with increase the boriding temperature and time Thick and compact boride layers could be obtained at temperatures higher than 900 °C The growth kinetics of boride layers is characterized by a parabolic curve FeB and Fe2B exhibit a hardness of 1600HV01 and 1300HV01, respectively, which is 4 and 3 times that of the substrate A diffusion model, based on the boron concentration profiles of the surface layers and the parabolic growth law, was established to predict the growth kinetics of the boride layers including both the FeB and the Fe2B layer A satisfactory agreement between the model and experimental results was obtained The work thus provides an approach to investigate and to predict the boride layer growth of 4Cr5MoSiV1 steel in the boriding process

Growth kinetics of the FeB/Fe2B boride layer on the surface of 4Cr5MoSiV1 steel: experiments and modelling

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M6C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M6C and martensite caused by the inconsistency deformation, M6C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness.

Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

In-depth understanding of the impact mechanism of nitrogen on tempering behavior of hot-working die steel is still limited. In present work, the microstructural evolution and mechanical property change of new nitrogen-alloyed Cr–Mo–V hot-working die steel during long-term tempering were investigated by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), eletron backscattered diffraction technique (EBSD) and 3D atom probe tomography (3DAP). The impact mechanism of nitrogen on tempering stability were discussed in detail. The results reveal that whether nitrogen is beneficial to the tempering stability of steel varies with quenching temperature. Trace nitrogen exerts a significant effect on the allowed quenching temperature and metastable M3C carbides, affecting the coarsening rate and transformation of M23C6 carbides during tempering process. Tempering stability was improved by nitrogen mainly by enhancing precipitation hardening and grain refinement strengthening, which account for about 70% of total yield strength. Improving effect of nitrogen in precipitation hardening is more distinct with the increase of tempering time. These results are helpful for understanding the mechanisms of nitrogen in steels worked at high temperatures.

Microstructural evolution and mechanical property changes of a new nitrogen-alloyed Cr–Mo–V hot-working die steel during tempering

It is known that mechanical properties of hot working die steel deteriorate during service due to dynamic recovery, recrystallization and plastic deformation of tempered martensite. In order to clarify the associated microstructure evolution and fracture mechanism, a systematic investigation was carried out on a widely used hot working die steel. Samples were tensile deformed at different temperatures ranging from 25 ℃ to 640 ℃ and examined using SEM and TEM. Results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature. These phenomena have been elaborated together with microstructure evolution during high temperature deformation. Deformation mechanisms and roles of different carbides in crack formation and propagation have been discussed.

Microstructure evolution and fracture mechanism of H13 steel during high temperature tensile deformation

High-temperature softening mechanism and kinetic of 4Cr5MoSiV1 steel during tempering

Hardness and failure assessment of a hot extrusion punch during service

The failure mechanisms under thermal-mechanical conditions have always been a critical issue in hot work steels. However, previous studies mainly focused on the mechanical properties of hot work materials, while the underlying deformation mechanism remains largely unclear. Here, we investigate the crack morphology and microstructure evolution mechanism near the fracture surfaces of 4Cr5MoSiV1 hot work die steels subjected to the uniaxial tension at 580 °C. An evident deformation band was observed consisting of refined α-Fe grains with a width of about 100nm along both sides of the intergranular fracture surface. Additionally, characteristic slip band rings formed frequently, presumably due to the grain rotation near the crack. Finally, carbides (including MC, M7C3 and M23C6) were also considered as possible crack nucleation sources due to the existence of incoherent boundary between these carbides and the ferrite matrix.

Microstructure evolution mechanism near the fracture lip of 4Cr5MoSiV1 steel during deforming at 580°C

The invention relates to a hot working die thermo-mechanical fatigue tester. The tester comprises a hot working die mounting base and a preform block mounting base. The tester also comprises a power mechanism for driving the hot working die mounting base and the preform block mounting base to carry out motion towards each other in the opposite directions and motion far from each other in the opposite directions. The tester also comprises a heating device for heating a preform block on the preform block mounting base. The heating device is used for heating the preform block and then the power mechanism drives the hot working die to carry out ejection-pressing fit to the preform block so that heat conduction between the hot working die and the preform block is realized and thus the hot working die is heated and the real working condition of the hot working die in a mechanical stress cycle and a temperature cycle is simulated. The tester realizes an alternating mechanical stress state and an alternating heat load state of the hot working die under actual motion conditions and has the characteristics of high reliability, good stability and measured value close to the real value.

Yaoli Wang

Papers

Microstructure evolution and fracture mechanism of H13 steel during high temperature tensile deformation

High-temperature softening mechanism and kinetic of 4Cr5MoSiV1 steel during tempering

Hardness and failure assessment of a hot extrusion punch during service

Microstructure evolution mechanism near the fracture lip of 4Cr5MoSiV1 steel during deforming at 580°C

Hot working die thermo-mechanical fatigue tester